Active matrix liquid crystal display devices with feedback control of drive signals

ABSTRACT

An active matrix liquid crystal (LC) display device, comprising in a display area ( 25 ) an array of picture elements ( 12 ) each having a picture element electrode ( 15 ) which together with an opposing common electrode ( 24 ) defines an LC display element ( 21 ) and a storage capacitor ( 20 ) connected to the picture element electrode, includes adjustment means ( 40,34 ) for adjusting drive signals applied by a drive circuit ( 35 ) to the picture elements ( 12 ) in accordance with changes in the LC capacitance. The adjustment means comprises an oscillator circuit ( 40 ) which is coupled to at least some of the picture elements in the array and whose oscillation frequency is determined by a capacitance associated with those picture elements and dependent on the capacitances of their LC display elements. The oscillator circuit may be coupled, via switch means ( 50,61,72 ), to a storage capacitor line ( 22 ) interconnecting the storage capacitor ( 20 ) of the picture elements ( 12 ) or to the common electrode ( 24 ). The oscillator circuit may be integrated on a substrate of the device, together with the picture element drive circuitry ( 35 ).

This invention relates to active matrix liquid crystal display devices(AMLCDs) and more particularly to AMLCDs having in a display area anarray of picture elements operable to produce a display image, eachpicture element comprising a picture element electrode, which togetherwith an opposing, common, electrode defines a liquid crystal displayelement, and a storage capacitor connected to the picture elementelectrode, and including adjustment means for adjusting drive signalsapplied to the picture elements in accordance with changes in the liquidcrystal capacitance.

Techniques for automatically controlling display drive parameters, forexample, the DC bias appearing across the display elements in an AMLCD,are known. However, such techniques are often complicated and difficultto implement. In order to provide automatically adjustments to the drivesignals using a feedback type control circuit it is necessary toidentify a way of determining how the liquid crystal (LC) material isaffected by the drive voltages applied to it. A preferablecharacteristic in this respect is the capacitance of the LC material.The capacitance of the LC layer is related to the orientation of the LCmolecules and therefore is closely related to the optical behaviour ofthe LC display elements.

W001/91427 describes an LC display device in which pairs ofinterconnected dummy LC display elements located outside the displayarea are driven in a particular way, shorted together and the resultingvoltage measured with a sense amplifier, this voltage being anindication of response time or the clearing temperature of the LCmaterial. However, these techniques normally require the use of analoguecircuitry and therefore are unsuitable in, for example, display deviceshaving integrated drive circuitry fabricated using the same thin filmtechnology as for the picture element array, such as poly-Si AMLCDsusing polycrystalline silicon type thin film transistors (TFTs) asswitching devices in the picture elements. For such purposes, it wouldbe desirable to use a technique which can be implemented using simplecircuitry that can easily be integrated onto a substrate of the displaydevice using TFTs, thereby minimising the external circuitry required tooperate the display and avoiding the need to adjust individually thedrive conditions of each display.

It has also been proposed in WO 01/91427 to use in an AMLCD for thepurpose of sensing the clearing point of the LC material a single dummyLC display element located outside the area of the picture element arraywhich is connected to an oscillator circuit and whose capacitance is oneof the parameters that determines the frequency of oscillation. Thisapproach may be acceptable for sensing simply the clearing point, but isunsuitable for measuring other operational characteristics of thedevice, particularly those associated with the behaviour of the actualdisplay elements, for example the response of the LC material to appliedvoltage or temperature changes. For this the dummy display element wouldneed to be truly representative of the actual display elements in allaspects of their behaviour, which would be difficult to achieve inpractice. Amongst other things, the effect of stray capacitance of themeasurement circuit, or the connections to it, on the operation of thesingle dummy display element would probably make it impossible to usethe dummy display element for such measurement purposes.

According to the present invention, there is provided an AMLCD asdescribed in the opening paragraph, wherein the adjustment meanscomprises an oscillator circuit which is coupled to a plurality ofpicture elements in the array and whose frequency of oscillationprovides a measure of a capacitance associated with the plurality ofpicture elements and dependent on the capacitance of their LC displayelements.

The invention results in significant advantages. Because the adjustmentmeans uses actual picture elements in the display area, the difficultyof creating dummy display elements that are truly representative of theactual display elements in all aspects of their behaviour is avoided.The measurement will take into account different drive conditionsexperienced by the display elements over time, for example resultingfrom different video images being displayed over prolonged periods,without necessarily requiring the generation of any special drivesignals. Moreover, the result of the measurement performed by theadjustment means would be representative of the average drive conditionsexperienced by the LC display elements of the picture elements usedtaking into account variations over the area of the picture elementarray, for example in alignments or dielectric layer thicknesses, andconsequential non-uniformities.

Further, because the adjustment means measures a capacitance to which aplurality of picture elements contribute, rather than the capacitance ofa single display element, the effect of stray capacitance of theadjustment means, or connections to it, is avoided or at leastconsiderably reduced. Importantly, unlike the approach used in theearlier proposal, the invention does not rely on the direct measurementof an individual LC display element capacitance. Further, the techniqueused in the invention is entirely compatible with utilising pictureelements in the array and does not require, for example, picture elementelectrodes to be connected together in a particular fashion. In thisrespect, the adjustment means is preferably arranged to measure thecapacitance associated with either the common electrode or the storagecapacitors of the plurality of picture elements, in the latter casepreferably via the storage capacitor line(s) conventionally used toconnect the storage capacitors of a row of picture elements together. Itwill be appreciated that the capacitances of the storage capacitors andcommon electrode depend on the LC display element capacitances, andtherefore can provide an indirect means of determining the capacitance,and hence the state, of the LC display elements without requiring aspecial display element lay-out.

By using in the adjustment means an oscillator circuit in which thecapacitance measured determines (at least in part) the frequency ofoscillation provision of the adjustment means is simplified. Such anoscillator circuit can be readily implemented using simple circuitry,for example with CMOS logic gates, and easily integrated onto the activesubstrate of the device using thin film circuit components comprisingTFTs, for example poly-Si TFTs, thereby minimising the externalcircuitry required and avoiding the need to individually adjust thedrive conditions of each display device. The frequency of oscillation ofthe circuit provides a measure of the response of the LC to factors suchas applied voltages and ambient temperature. The output signal from thecircuit, indicative of the frequency, can readily be used to implementautomatic adjustment of one or more of the parameters of the drivewaveforms employed.

In order to avoid, or at least minimise, disturbance to the displayimage produced by the picture elements, for example as a result of theperformance of a measuring operation possibly affecting the voltagewaveforms appearing across the LC, then the measurement by theadjustment means may be applied to all the picture elements in thearray, either simultaneously or in groups. Accordingly, this will avoidbanding or blocking effects in the display image which may otherwise beapparent if, for example, only a few rows of picture elements were to beused for measurement purposes.

Switch means is preferably included which is selectively operable toswitch the common electrode or storage capacitor connection line betweena potential source and the oscillator circuit.

Any possible disturbance of the display elements voltage is likely tooccur only while a measurement is being made. The time required toperform a measurement can be made very small in comparison with theframe period so that display element disturbance will have only a verysmall effect on the rms voltage or average voltage appearing across thedisplay elements.

Embodiments of AMLCDs in accordance with the present invention will nowbe described, by way of example, with reference to the accompanyingdrawings, in which:

FIG. 1 shows the equivalent circuit of a conventional AMLCD;

FIG. 2 illustrates in block diagram the operating principle of an AMLCDin accordance with the invention;

FIG. 3 shows schematically an example circuit employed in adjustmentmeans used in an AMLCD according to the invention;

FIG. 4 illustrates example waveforms present in operation of the circuitof FIG. 3;

FIG. 5 is a graph illustrating the relationship between certain drivevoltages and the output of the circuit of FIG. 3;

FIG. 6 is a graph illustrating the relationship between a commonelectrode voltage of AMLCD and the output of the circuit of FIG. 3;

FIG. 7 shows the equivalent circuit of a first embodiment of AMLCDaccording to the present invention;

FIG. 8 illustrates an alternative arrangement in the AMLCD of FIG. 7;and

FIG. 9 shows the equivalent circuit of a second embodiment of AMLCD inaccordance with the present invention.

It will be appreciated that the figures are all schematic. The samereference numbers and symbols are used throughout the figures to denotethe same or similar parts or features.

Various embodiments of AMLCDs in accordance with the invention will bedescribed. The construction and general operation of these devicesfollow conventional practice and will not be described here in detail.For further information in these respects reference is invited, forexample, to US-A-5130829 which describes the basic operational andconstructional principles of an AMLCD.

The circuit configuration of a typical AMLCD is shown schematically inFIG. 1. The device comprises a row and column matrix array of pictureelements 12 located at respective intersections between crossing sets ofrow and column address conductors 14 and 16. Each picture element has aTFT (thin film transistor) 18 whose drain electrode is connected to apicture element electrode 15 and whose gate and source electrodes areconnected to a row conductor 14 and column conductor 16 respectively.The gates of the TFTs in a row of picture elements 12 are connected tothe same row conductor 14 while the source electrodes of all TFTs in acolumn of picture elements are connected to the same column conductor16. Each picture element 12 further includes a storage capacitor 20connected between the picture element electrode 15 and a respectivecapacitor line 22 shared by a row of picture elements. The capacitorlines 22 for all rows in the array are connected at their ends to apredetermined reference potential source 23, for example, ground. Theconductors 14 and 16, TFTs 18, picture element electrodes 15, storagecapacitors 20 and lines 22 are all carried on an insulating firstsubstrate (not shown), for example of glass. A second substrate, forexample also of glass, spaced form the first substrate carries anelectrode layer 24, typically of ITO, common to all picture elements 12in the array. LC material is disposed between the two substrates andeach picture element electrode 15 together with the immediatelyoverlying portion of the common electrode 24 and the LC materialsandwiched therebetween constitutes an LC display element 21. The twosubstrates together with the LC material sealed inbetween form an LCcell structure.

The array of picture elements 12 defines a display area 25 (here thearea denoted by a dotted line) in which a display image is produced inoperation. The rows of picture elements 12 are addressed one at a timein sequence by means of a row drive circuit 28 which applies to each rowconductor 14 in turn a selection (gating) signal in a respective rowaddress period which turns on the TFTs 18 of the row. A column drivecircuit 30 applies data signals to the column conductors 16, obtained bysampling an input video signal, in synchronism with row addressing suchthat the picture element electrodes 15 in a selected row are charged,via the TFTs, according to the level of the voltage of the data signalson the respective column conductors 16. The drive voltage applied to apicture element electrode 15 determines a desired display effect withthe light transmission through the display element 21 being modulatedaccording to the level of the applied drive voltage to produce a displayoutput ranging from fully on (white) to fully off (black) throughintermediate grey-scale levels. At the end of the row address period,following termination of the selection signal, the TFTs of the row areturned off to isolate the electrodes 15 and the applied voltage isstored on the display element capacitances and their associated storagecapacitors 20 until they are addressed again, usually in the next frameperiod. Each row of picture elements is addressed in turn so as to buildup a complete display picture over one frame and the array of pictureelements repeatedly addressed in this manner in subsequent frameperiods.

In each of the AMLCD embodiments to be described adjustment means in theform of a feedback control circuit is used to provide automaticadjustment of, for example, the picture element drive waveforms forvarious purposes, as will be explained. To this end the capacitance ofthe LC layer at the display elements of the device is utilised as ameans of determining the effect on the LC of applied drive waveforms,e.g. voltages and timings, the LC capacitance being related to theorientation of the LC molecules, and thus closely related to the opticalbehaviour of the LC display elements. In these embodiments an oscillatorcircuit is employed in the adjustment means and the capacitance of theLC provides one of the parameters which determines the frequency ofoscillation. This frequency therefore provides a measure of the responseof the LC to factors such as applied voltage and temperature.

To obtain the LC capacitance, a group of picture elements, a number ofgroups of picture elements, or all the picture elements in the pictureelement array may be utilised so as to provide an average capacitancemeasurement.

The general scheme of the operation of the embodiments of AMLCD with theadjustment means is shown in the block diagram of FIG. 2, in which theblock 35 represents the array drive circuit, which includes the row andcolumn driver circuits 28 and 30, the block 36 represents the array ofpicture elements 12. The drive circuit 35 is arranged to producerequired LC drive voltage waveforms across the LC display elements.These waveforms may typically be similar to those used conventionally.

A display control circuit, 34, provides the necessary timing and controlsignals for the array drive circuit 35 and also the video signal VSsupplied to the circuit 34 from an external video source and from whichthe data signals for the picture elements are derived. Picture elementsin the array are connected to the oscillator circuit, here denoted bythe block 40 through a coupling circuit 38. The function of the circuit38 is to couple the capacitance of the LC display elements 21 of thepicture elements 12 to the input of the oscillator circuit 40 so thatthe frequency of oscillation depends on the display element capacitancewhilst limiting the extent to which the operation of the circuit 40might affect the voltages across the display elements 21 and alsolimiting the direct effect that the drive waveforms applied to thedisplay elements have on the operation of the circuit 40. The drivewaveforms will, of course, affect the frequency of oscillationindirectly by way of changing the capacitance of the LC displayelements.

FIG. 3 shows schematically an example implementation of the coupling andoscillator circuitry in more detail. For simplicity, here only one LCdisplay element 12 is shown although in practice a plurality of displayelements would be used. The liquid crystal drive circuit 35 includes asource of an alternating voltage waveform D (e.g. the LC data signaldrive waveform from the column drive circuit 30) and a switch S₁,comprising a TFT 18, which allows the LC display element to beperiodically charged, (according to a switching signal S controlling theswitch S₁) to the level of the LC drive waveform D. Connected inparallel with the capacitance of the LC display element, C_(LC), is thestorage capacitor 20 with capacitance C_(S). The second terminal of thestorage capacitor is connected to ground via a switch, S₂. The input ofthe oscillator circuit 40 is coupled to the display element 21 by theseries connected capacitors C_(C) and C_(S). The oscillator is formedusing a CMOS inverter 45 with a resistor 46, R_(OSC), providing feedbackfrom the output of the inverter to the input. The output of theoscillator is buffered by a second inverter 47. When the switch S₂ isclosed the capacitance at the input of the oscillator is approximatelyequal to C_(C). When a measurement indicative of the capacitance of theLC display element 21 is to be made switch S₂ is opened. The capacitanceat the input of the oscillator is then approximately(1/C_(LC)+1/C_(S)+1/C_(C))⁻¹. The frequency of oscillation of thecircuit therefore depends on the value of C_(LC).

Waveforms which illustrate how this circuit might be operated are shownin FIG. 4. The LC display element drive signal waveform D consists of aframe inversion data signal voltage waveform which changes polarityevery 16.6 ms (for a VGA display). The select waveform, S, causes theswitch S₁ to close once in every 16.6 ms period which results incharging of the storage capacitor 20 and the LC display element 21capacitance to the LC drive signal voltage. LCE is the voltage acrossthe LC display element 21. When the capacitance of the display element21 is to be measured the measurement enable waveform M applied to theswitch S₂ goes high causing switch S₂ to open. The duration of themeasurement enable pulse in this example is set at 1 ms. The oscillatoroperates continuously and when the measurement enable signal is low thefrequency of oscillation depends principally on the value of thecapacitance C_(C). When the measurement enable signal is high thefrequency of oscillation depends on the value of the series combinationof C_(LC), C_(S) and C_(C), that is (1/C_(LC)+1/C_(S)+1/C_(C))⁻¹. Theoscillator circuit output waveform, comprising a succession of outputclock pulses, is shown at OS. This signal is fed back to the displaycontrol circuit 34 where it can be used to provide adjustments to drivewaveforms for various different purposes. During the measurement processa small signal will be coupled from the input of the oscillator circuitonto the LC display element 21. However this will have relatively littleeffect on the behaviour of the liquid crystal because of its lowamplitude and relatively short duration. A measure of the capacitance ofthe LC display element 21 can be obtained by counting the number ofcycles of the oscillator output during the 1 ms period when themeasurement is enabled. The oscillator frequency provides aninstantaneous measurement of the capacitance of the LC display element21. In this example, the measurement is enabled (M) some time after thechange in the polarity of the drive voltage applied to the liquidcrystal in order to allow for the response time of the liquid crystalmolecules.

FIG. 5 shows measured results illustrating the way the number, N, ofoscillator clock periods during the measurement period of 1ms varieswith the peak to peak drive voltage, P, applied to the LC displayelements by the liquid crystal drive circuit 35. When the drive voltageis low the capacitance of the LC element is relatively low and thereforethe frequency of oscillation and the count of the oscillator clockperiods is relatively high. As the drive voltage is increased the liquidcrystal molecules start to react to the applied voltage by changingtheir orientation which results in an increase of the capacitance of theLC element. This causes the capacitance at the input of the oscillatorto increase which in turn causes the oscillator frequency and the countof oscillator clock periods to fall. As the drive voltage is increasedfurther the movement of the liquid crystal molecules start to saturateso that the capacitance of the LC element tends towards a maximum valueand the oscillator clock frequency tends towards a minimum value.

The variation of oscillator frequency with drive voltage which is shownin FIG. 5 gives an indication of the response of the liquid crystal tothe applied peak to peak drive voltage and can therefore be used in thedisplay control circuit 34 to provide automatic adjustment of the drivevoltage waveforms of the display device. For example, changes in thebehaviour of the liquid crystal as the ambient temperature of thedisplay varied can be detected using this technique. This might involvedetermining the threshold voltage of the liquid crystal by detecting thedrive voltage at which the capacitance of the liquid crystal starts toincrease from its minimum value (the point at which the oscillatorfrequency starts to fall from its maximum value). Knowledge of thethreshold voltage of the liquid crystal could then be used to determinethe drive voltages required by the display device. In a more advancedscheme the measured capacitance versus drive voltage behaviour of theliquid crystal might be used to determine the gamma correction appliedto the display device. This might be carried out by using thecapacitance information to generate data for a look up table or by usingit to select one of a number of predetermined gamma functions.

Another aspect of establishing the correct drive voltages for a liquidcrystal display device is minimising the dc voltage across the liquidcrystal. If the dc voltage applied to the LC display elements 21 is notset correctly then problems such as low frequency flicker and imagesticking can occur. By comparing the capacitance of the LC displayelement resulting from positive and negative drive voltages it ispossible to determine when the dc voltage across the liquid crystal iscorrectly adjusted. FIG. 6 shows the effect that varying the dc voltageapplied to the common electrode 24 of the LC display element 2 has onthe oscillator frequency during periods when the LC element receivespositive and negative drive voltages. In this Figure, CE is the commonelectrode voltage, N is, again, the number of oscillator clock periodsin 1 ms, and NDP and PDP are respectively the negative drive period andthe positive drive period.

When the common electrode potential is correctly adjusted the voltageacross the liquid crystal is equal in magnitude but opposite in polarityduring positive and negative drive periods. Therefore the capacitance ofthe LC display element 21 and the frequency of the oscillator will bethe same for the positive and negative drive periods. When the dcvoltage on the common electrode 24 is made more negative than itsoptimum value the voltage across the liquid crystal is increased duringthe positive drive periods and decreased during the negative driveperiods. As a result, the capacitance of the liquid crystal is increasedduring the positive drive period and decreased during the negative driveperiod. This is reflected in the decreased oscillator frequency duringthe positive drive period and the increased frequency during thenegative drive period which can be seen in FIG. 6. When the commonelectrode potential is made more positive than its optimum value thechanges are reversed.

There is an increase in the oscillator frequency during the positivedrive period (PDP) and a decrease in the frequency during the negative(NDP) drive period. Using the output signal OS, therefore, the dcvoltage appearing across the LC display element can accordingly beminimised by adjusting the dc potential of the common electrode 24 untilthe difference between the oscillator frequencies in positive andnegative drive periods is minimised.

Referring again to FIG. 2, the output from the oscillator circuit 40 isfed back to the display control circuit 34. This circuit applies drivesignals to the picture element array via the drive circuit 35 andmeasures this response by determining the output frequency of theoscillator circuit 40. The circuit 34 controls the characteristics ofthe drive signals applied to the array and uses the information obtainedfrom the measurement of the LC display elements'capacitance to ensurethat the applied drive waveforms are correctly adjusted. Suitablecircuits for modifying or adjusting the drive signals for the purposesdescribed previously will be apparent to persons skilled in the art.

The oscillator circuit 40 may be used to measure the capacitance of theliquid crystal in a number of ways. For example, it might be operatedcontinuously so that the frequency of oscillation provides an indicationof the way in which the capacitance of the liquid crystal cell structurevaries with time. Alternatively, the oscillator circuit might beoperated at specific times, effectively sampling the value of the liquidcrystal capacitance. The drive voltage applied to the liquid crystalmight be stepped through a number of values and the capacitance measuredfor each value in order to characterise the response of the liquidcrystal to drive voltage. Other characteristics of the drive signalsmight be varied and the response of the liquid crystal measured, forexample the response to a change in drive frequency or addressingfrequency might be measured.

Example embodiments of AMLCDs in accordance with the present inventionusing the above described type of adjustment means will now bedescribed. In these embodiments, all the LC display elements 21 in thedisplay area array are utilised by the adjustment means. The possibilityof unwanted display artefacts which could occur when using only selecteddisplay elements for this purpose is then avoided. However, only some ofthe display elements may be utilised if desired. In these embodiments,the oscillator circuit 40 is arranged to measure a capacitance which isassociated with the display elements and which is dependent on the LCdisplay element capacitances rather than measuring an LC display elementcapacitance directly. The capacitor lines 22 or the common electrode 24are utilised for this purpose. As actual picture elements in the arraydisplay area are used rather than dummy picture elements, themeasurement consequently takes into account the different driveconditions experienced by the picture elements, for example resultingfrom different video images being displayed over time, without requiringthe generation of any special drive signals. The result of themeasurement will be representative of the average drive conditionsexperienced by the picture elements taking into account also variationsover the area of the array due to variations in alignments or dielectricthicknesses.

Referring to FIG. 7, the circuit configuration of a first embodiment ofAMLCD in accordance with the invention is shown in which the capacitorlines 22 are used to provide the input to the oscillator circuit 40. Thedevice is in most respects similar to that of FIG. 1. The capacitorlines 22 are all interconnected together at their one ends and, again,connected to the low impedance, reference potential source 23, except inthis case via a switch, 50, corresponding to the switch S₂ in the FIG. 3circuit arrangement. The lines 22 are connected also to the input of theoscillator circuit 40 via the coupling capacitor C_(C), as in the FIG. 3circuit arrangement.

When a measurement is being made, the switch 50 connecting the lines 22to the low impedance source 23 is opened so that the capacitance of thedisplay elements 21 becomes one of the parameters determining thefrequency of oscillation of the circuit 40.

The polarity of drive voltages applied to the LC display elements 21usually needs to be inverted periodically. Conventionally, thisinversion may be every frame. However, in some schemes, for example aline inversion drive scheme in which the polarity of drive voltages isinverted for successive rows, the nature of the addressing of thepicture elements may be such that half the display elements areaddressed with a positive drive voltage and half the display elementsare addressed with a negative drive voltage. If it is necessary toseparately measure the response of the LC display elements 21 to thesetwo drive polarities then it will be necessary to provide separateconnections to display elements receiving positive and negative drivevoltages. For example, if the array is addressed using a row inversiondrive scheme in which alternate rows of picture elements are addressedwith opposite polarities then the capacitor lines 22 of the alternaterows could be joined to common points and a switching arrangement usedto connect the elements of one of the two sets of rows to the input ofthe oscillator. This would allow a capacitance measurement to be made ondisplay elements receiving positive drive voltages and display elementsreceiving negative drive voltages within the same frame period.

As mentioned earlier the finite response speed of the liquid crystalmolecules means that when the drive voltage applied to the liquidcrystal is changed it takes some time for the liquid crystal to respondto this change. In the measuring scheme indicated previously (FIG. 4)the capacitance measurement is made shortly before the voltage acrossthe LC display element 21 is inverted in order to ensure that the liquidcrystal had been given time to react to any change in voltage. A similarapproach can be implemented when the capacitance associated with thedisplay elements is being measured by using the coupling arrangement inthe alternative device circuit illustrated in FIG. 8. In this case acapacitor line selector circuit 60 selectively controlling a group ofchange-over switches 61 is used to determine which of the capacitorlines 22, or groups of capacitor lines, is connected to the input of theoscillator circuit 40 at any one time. The switching of the capacitorline selector circuit 60 can be synchronised with the operation of therow drive circuit 28 of the device to ensure that the capacitancemeasurements are made at an appropriate time within the addressing cycleof each row of picture elements.

The description above indicates the general manner in which anoscillator circuit is used to measure the capacitance of LC displayelements in order for example to determine their response to the applieddrive waveforms. A specific example of an oscillator circuit 40 has beendescribed (FIG. 3) which is particularly simple and suitable forintegration onto the first substrate of the AMLCD using thin filmtransistors. Other types of oscillator circuit could also be used in asimilar way as long as the changing capacitance of the LC displayelements is one of the parameters which determines their frequency ofoscillation. In the examples given it is convenient to operate theoscillator circuit 40 continuously although it would clearly be possibleto enable the oscillator circuit only during a measurement period inorder to minimise the power consumption of the AMLCD. In the embodimentsof FIGS. 7 and 8 the coupling of the input of the oscillator circuit tothe LC display elements is achieved by making use of the storagecapacitors which are normally connected in parallel with the LC displayelements and by adding a further coupling capacitor, C_(C). There willalso be other methods by which the capacitance of the LC elements 21could be coupled to the input of the oscillator, as will be apparent topersons skilled in the art. One such other method would be to use thecommon electrode 24 of the AMLCD to provide this connection rather thanthe capacitor lines 22.

FIG. 9 shows the circuit configuration of a second embodiment of AMLCDaccording to the invention using the common electrode 24 to provideinput to the oscillator circuit 40. This example also uses analternative configuration for the storage capacitor in which separatecapacitor lines are not provided and instead the sides of the storagecapacitors 20 remote from the picture element electrodes 15 areconnected to a row address conductor 14 of an adjacent row of pictureelements 12.

The common electrode 24 is connected to a common electrode drive circuit70 via a switch 72 which corresponds functionally with the switch S₂ inthe FIG. 3 circuit arrangement and to which the measure enable waveformM is applied. The common electrode 24 is connected also to the input ofthe oscillator circuit 40 via the coupling capacitor C_(C). In otherrespects, the circuitry and the operation of the adjustment means issimilar to that of the previous embodiment.

In the above example embodiments, a single oscillator circuit is used tomeasure the capacitance of different LC display elements 21. This isimportant when a direct comparison of the capacitance of the elements isrequired because the measured frequency will depend on thecharacteristics of the oscillator circuit. However, there may becircumstances where it is preferable to provide more than one oscillatorcircuit. Separate oscillator circuits could be provided for differentsets of LC display elements. For example, one oscillator circuit may beprovided for each row of picture elements in the case of the embodimentof FIG. 8.

The measurement of the capacitance of the LC display elements can beused to control the drive waveforms of the AMLCD as describedpreviously, for example to provide automatic adjustment of the drivevoltages applied to the picture elements, specifically the dc voltageappearing across the liquid crystal and the peak to peak drive voltagewhich determines the greyscale performance of the device. In principlethe approach can be extended to the automatic adjustment of any aspectof the display drive waveforms which cause the response of the liquidcrystal to change. For example the row select (gating) or non-selectvoltages of the waveform applied by the drive circuit 28 to the rowaddress conductors 14 could be adjusted by detecting whether a smallchange in these voltages has any effect on the capacitance (andtherefore on the grey-level) of the display elements within the array.As another example, in order to minimise the power consumption of theAMLCD, the addressing frequency could be reduced to a level determinedby detecting when a further reduction in frequency would result inunacceptable discharge of the display elements during the frame period.The discharge of the display element voltage could be detected via thechange in the capacitance of the liquid crystal. This measurementtechnique could also be used to determine the switching speed of theliquid crystal and to adjust a correction algorithm.

Some of these measurements and adjustments of the drive waveformparameters could be carried out at extended intervals, for example, eachtime that the AMLCD is turned on. Ideally, the values of the parameterswould be stored so that it was only necessary to make small adjustmentsto the drive parameters when the device is turned on rather than havingto establish the parameters from some default setting. Thesemeasurements might require some specific test waveforms or test patternsto be applied to the AMLCD or the LC display elements 21 during thetest. For example signals representing different grey levels might beapplied, the drive frequency might be altered, or some other aspect ofthe drive conditions varied.

Other measurements might be performed during the operation of the AMLCD.For example, adjustment of the drive voltages to correct for the effectof temperature variations might be carried out periodically while thedevice is operating.

It may be advantageous to integrate a number of separate LC displayelement capacitance measurement circuits on the devices substrate. Thesecould control different aspects of the drive waveforms applied to thearray and they could be designed and operated in a way which is bestsuited to their function. For example LC display elements 21 within thearray could be used to determine the dc voltage applied to the array.One of the parameters which determines the dc voltage is the offsetvoltage which occurs within the picture elements 12 when the TFTs 18turn off. It is therefore advantageous to adjust the dc voltage bymeasuring the capacitance of the display elements within the array.

The proposed form of adjusting means is most relevant to AMLCDs in whichthe drive circuits are integrated onto the active substrate of thedevice. However, this adjustment means and measurement technique couldalso be implemented using external circuitry, for example within thecrystalline silicon drive ICs of an AMLCD which does not have integrateddrive circuits.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the field of active matrixliquid crystal display devices and component parts therefore and whichmay be used instead of or in addition to features already describedherein.

1. An active matrix liquid crystal display device having in a displayarea an array of picture elements operable to produce a display image,each picture element comprising a picture element electrode whichtogether with an opposing, common, electrode defines a liquid crystaldisplay element, and a storage capacitor connected to the pictureelement electrode, the device including adjustment means for adjustingdrive signals applied to the picture elements in accordance with changesin the liquid crystal capacitance wherein the adjustment means comprisesan oscillator circuit which is coupled to a plurality of pictureelements in the array and whose frequency of oscillation provides ameasure of a capacitance associated with the plurality of pictureelements and dependent on the capacitance of their respective liquidcrystal display elements.
 2. A device according to claim 1, whereinrespective first electrodes of the storage capacitors of the pluralityof picture elements are connected together and wherein the adjustmentmeans is arranged to measure the capacitance of the connected firstelectrodes of the storage capacitors.
 3. A device according to claim 1,wherein the storage capacitors are connected between their respectivepicture element electrodes and a connection line common to the storagecapacitors of the plurality of picture elements and wherein theadjustment means is arranged to measure the capacitance associated withthe connection line.
 4. A device according to claim 3, wherein thestorage capacitor connection line is connected to switch means that isselectively operable to couple the connection line to a source ofpredetermined potential or to the oscillator circuit to enable theadjustment means to perform a measuring operation.
 5. A device accordingto claim 1, wherein the adjustment means is arranged to measure thecapacitance of the common electrode.
 6. A device according to claim 5,wherein the common electrode is connected to switch means that isselectively operable to couple the common electrode to a source ofpredetermined potential or to the oscillator circuit to enable theadjustment means to perform a measuring operation.
 7. A device accordingto claim 1, wherein the oscillator circuit of the adjustment means iscoupled to all the picture elements in the array with the measurementprovided thereby being dependent on a capacitance associated with thedisplay elements of all the picture elements in the array.
 8. A deviceaccording to claim 1, wherein the oscillator circuit of the adjustmentmeans comprises thin film circuitry integrated on a substrate of thedevice which carries the picture element electrodes.
 9. A deviceaccording to claim 1, wherein an input of the oscillator circuit of theadjustment means is coupled to the plurality of picture elements via acoupling circuit comprising a capacitor.